My 2017 GT w/ premium package came pre-wired for paddle shifters within the steering stem, so there are no modifications outside of the steering wheel proper. The way I tested this is by shorting the corresponding shift paddle pins on the clock spring male connector with ignition on in manual shift mode. The dash reflected my +1 -1 gear choice, although at a standstill you are limited to just the first 2 gears.
Where it there is some work to be done is in the steering wheel. I used the following parts for my build:
B64E-32-049-02 - steering wheel core (the rear cover). This item takes crude modification with a sharp object around the center bottom spoke area to properly fit the wheel.
B64F-32-750 - paddle shifter affixing bolts (set of 4)
BHT1-66-3P0 - steering wheel-mounted shift paddle assembly
BNK8-66-4M2 - in-steering wheel main harness. This part is not used in whole, but as a donor for the 3-wire paddle shift sub-harness that is plugged into the harness coming out of the paddle shifters. I was unable to find the 3-pin to 5-pin female shifter sub-harness for sale by itself, which is why this larger harness was used.
You will have to drill an extra hole in the new rear cover at the 6 o’clock position so that the new 3 latch point airbag can be released, and steering wheel taken off in the future.
IF YOU DO NOT DO THIS, YOU WILL NOT BE ABLE TO TAKE OFF YOUR STEERING WHEEL WITHOUT DAMAGE TO IT. You can overlay your original backplate and drill the center airbag release hole. A small tip on unlatching the airbag: when inserting the screwdriver into either of the side latch release holes, start at the side and angle the screwdriver inward. It takes a bit of pressure on the plastic plate underneath to deform the aluminum wire used to latch the airbag onto the aluminum steering wheel core. You should always be feeling progressive resistance as you do this. If you find that an unstoppable force meets an unmovable object, you’re doing it wrong. The third latch point requires more of a straight-on pressure. The more problematic install is the one for those of us with heated steering wheels. The heated units contain an extra box and harness hidden away behind the 6 o’clock position black plate (as looking at the steering wheel), that is located between the two bottom spokes. For heated steering wheels with paddle shifters (like Mazda6 with premium package) there is a different rear steering wheel cover (or black plate) compatible specifically with these 2 features. I imagine it has more of a humped contour in the rear, like the original on the CX-5. I have exhaustively looked for this part online and with 3 separate dealerships, and it appears that Mazda has opted not to list it with its own p/n for sale, probably because they wised up about the DIY community adding shifters to cars that they equate to lost revenue. The backplate is, however, available as part of the complete Mazda6 heated, paddle shifted steering wheel (~$700) p/n: GRV1-32-98XD-02. This is where I did something crude. I took the B64E-32-049-02 backplate and cut into it a hole big enough to fit the heated steering wheel’s heater circuit assembly (black box). It does stick out of the rear of the steering wheel by about 1/4 of an inch and has a rather hard edge, but it is so close to the center of the rear of the steering wheel that I would have to intentionally reach back there with my pretty large hands for it to be a problem. Until Mazda releases a "w/paddle shift, w/heat" rear cover, a clean installation is, unfortunately, ruled out. I guess I really wanted those paddle shifters…
Here are a couple of useful videos I used to guide me in the very similar 2017 DIY:
https://www.youtube.com/watch?v=rNedbR77iZ8
https://www.youtube.com/watch?time_continue=283&v=Go0M_AZlrXU
Hope this helps.
Shout out to Putnam Mazda of Burlingame, Capitol Mazda of San Jose, and MazdaSwag.com for their parts diagrams research for this project.